In digital mobile systems it is possible to transmit, in addition to normal speech and data connections set up on traffic channels, short digital data messages or short messages that are sent on control and signalling channels of a system between the actual signalling. Mobile systems are generally provided with a short message service centre that enables transmission of short messages between the elements wishing to send and receive short messages via the mobile network. In case of a short message originating from a terminal equipment, a subscriber forms a short message with the keyboard of the terminal equipment and the terminal equipment transmits the short message on the signalling channel via the base station and the mobile services switching centre to the short message service centre. The short message service centre forwards the short message to another network or to a terminal equipment of another subscriber via the mobile network on signalling channels.
The use of short messages has become far more common and new applications are defined continuously. In addition to messages received from terminal equipments, a short message service centre may also receive messages in an electronic form from IP, X.25 and X.400 networks, for example. Some operators also provide services where a short message can be delivered to the short message service centre by facsimile or it can be dictated in the form of a voice message so that the operator converts the message into a short message form. For processing messages arriving from different sources, a short message service centre is provided with an associated gateway application or gateway applications by means of which a message is converted into a short message form to be transmitted via the mobile system.
A subscriber terminal equipment may receive short messages from more than one short message service centre. A terminal equipment usually stores as a default value a short message service centre to which a short message originating from the terminal equipment is directed. GSM specification 03.40 (Appendix 4) defines a reply path comprising the reply routine used in connection with short messages. The reply path enables a reply transmitted by a subscriber to be routed to the mobile station that transmitted the original message, via the short message service centre via which the message was originally delivered to the subscriber, instead of the short message service centre stored in the replying terminal equipment as a default value. By means of the routine the subscriber may reply to a short message without having to know the network address of the mobile station or the short message service centre that originally transmitted the message.
When a message delivered to a subscriber originates from a source situated outside the area of the short message service centre, the routing of the reply message to the original source by means of the reply path is not possible, however. The reply path returns the reply to the short message service centre that transmitted the message, but the short message service centre comprises no information required to route the message to an external network. In order to be able to reply to a message that arrives from an external network, the mobile subscriber must know itself the required routing data. This makes the use of messages complicated and, in practice, subscribers cannot be expected to have such information.
One possibility of routing messages originating from a terminal equipment to an external network is a database that is added to the short message service centre and that permanently stores a list of extra file names or private numbers that refer to the network addresses provided by the subscriber. However, this system is rather difficult and it increases the amount of data stored in the short message service centre, especially since the specifications must be stored in each short message service centre. The data must also be stored in the database before the transmission of a reply, and therefore the database is of no help in answering to a message arriving from a new source.